The Quran

Preface

وَتَحسَبُهُم أَيقاظًا وَهُم رُقودٌ ۚ وَنُقَلِّبُهُم ذاتَ اليَمينِ وَذاتَ الشِّمالِ ۖ وَكَلبُهُم باسِطٌ ذِراعَيهِ بِالوَصيدِ ۚ لَوِ اطَّلَعتَ عَلَيهِم لَوَلَّيتَ مِنهُم فِرارًا وَلَمُلِئتَ مِنهُم رُعبًا

Yusuf Ali

Thou wouldst have deemed them¹ awake, whilst they were asleep, and We turned them on their right and on their left sides: their dog² stretching forth his two fore-legs on the threshold: if thou hadst come up on to them, thou wouldst have certainly turned back from them in flight, and wouldst certainly have been filled with terror of them.³

Perhaps their eyes were open, even though their senses were sealed in sleep. They turned about their sides as men do in sleep.
The name of their dog is traditionally known as Qitmir, but see n. 2336 above.
This graphic picture of the sleepers explains the human mechanism by which their safely was ensured by God from their Pagan enemies.